The materials that are used to build musical instruments are important because they vibrate at a specific frequency–a natural frequency. All objects vibrate, but when the amplitudes of the vibration are large enough and the natural frequency is in the human range of hearing, the sound waves are audible. Some objects vibrate at a single frequency and produce a pure tone (like a flute). Others vibrate and produce more complex freqencies that have a mathematical relationship. The result is a rich sound (like in a tuba). For string instruments, changing the length of the strings while playing (such as with guitar strings) really affects the sound. It shortens the string, which changes the wavelength of the instrument and with that, the sound. For wind instruments, changing the tube is what changes the sound of the instrument. It changes the size of the air column in the instrument, which affects the wavelength and the frequency. Yes, the size of an instrument is an important consideration of designing said instrument. A musician must find a material that positively affects the speed of the material (frequency) and a length that makes the sound waves the instrument produces audible.